Everything we know so far about the 'Game of Thrones' prequels

There are currently five prequel series to "Game of Thrones" in the works, each with a writer attached

One series, seemingly called "The Long Night," is about Westeros' descent from its "Age of Heroes into its darkest hour"

So far, Naomi Watts and Josh Whitehouse have been cast in major roles

Anywhere between "zero and five" of these series will eventually make it to television

For "Game of Thrones" fans already sad about the series' end after its next and final season, HBO and author George R.R. Martin aren't abandoning Westeros just yet.

While it's been known for over a year that HBO currently has five "Game of Thrones" prequel series in the works, at least one of them seems to be coming to fruition.

The series which Martin has begun referring to as "The Long Night" has announced the casting of Naomi Watts, which suggests that at least one of the five potential prequels is moving towards production. A day later, The Hollywood Reporter announced that Josh Whitehouse, who's slated to star in the remake of "Valley Girl," has been cast in a "key role."

Noami Watts has been cast in "The Long Night." Jason Merritt/Getty Images

Martin tweeted earlier this week announcing Watts' casting and updating fans on the status of the prequel series: "Casting is now underway for THE LONG NIGHT, the first of the GAME OF THRONES successor series to ordered to film." His referring to "The Long Night" as the "first" successor series to film implies that other series are still in development.

"The Long Night" will cover the dark history of Westeros

Variety reports that Jane Goldman, who is best known for co-writing "Kingsman: The Secret Service" and "Kingsman: The Golden Circle," wrote the new series with Martin, wrote the teleplay, and will be the series' showrunner.

Numerous outlets have been reporting the following description from the network: "Taking place thousands of years before the events of 'Game of Thrones,' the series chronicles the world's descent from the golden Age of Heroes into its darkest hour. And only one thing is for sure: From the horrifying secrets of Westeros' history to the true origin of the white walkers, the mysteries of the East, to the Starks of legend … it's not the story we think we know."

Beyond this description, all that's known so far about the series — which Martin has been referring to as "The Long Night," but the title of which has not been officially confirmed and could just be a working title - is that Watts plays "a charismatic socialite hiding a dark secret." No description has been released for Whitehouse's character yet, other than that he is not the central male lead.

Josh Whitehouse is also in "The Long Night." IMDB

HBO has a strong reputation for keeping juicy series details under wraps, so it may be a while before we know much more about the prequel. Early this year, HBO programming president Casey Bloys told The Hollywood Reporter that the first prequel series would not air until at least a year after the main show's finale - which means mid-2020 at the very earliest.

Very little is known about the other series in development

All that has been announced about the other series currently in development are the writers attached: Max Borenstein, Brian Helgeland, Carly Wray, and most recently, Bryan Cogman, as first reported by Entertainment Weekly. Cogman has written several episodes of "Games of Thrones" to date.

But what are these other potential prequels about? Given the vast and intricate universe Martin has created, the possibilities are virtually limitless. The only hints Martin has given so far are that they're unlikely to involve the characters Dunk and Egg (Ser Duncan the Tall and Aegon the Unlikely), or to cover the events of Robert's Rebellion - fair enough, as the main series addresses it pretty thoroughly.

There has, however, been one other clue. On his "Not a Blog," Martin writes: "Meanwhile, there are still a couple of other possible prequels in active development. I can't tell you the subject matter of those projects, no, sorry, wish I could. The readers among you might want to grab a copy of 'FIRE & BLOOD' when it is released on November 20, though."

"Fire and Blood" is a history of the Targaryen clan taking place 300 years before the main series - Martin's hint would suggest that one of the TV series in development covers this story arc. That is unless it's just some sneaky book promotion. We won't know until Martin and HBO want us to.